one more point

okay, so if there should be a minimum age for people to buy weed (if its morally wrong, and it were to become legal, then im sure youll want a minimum age for people to buy it), then how does one decide what age that should be? i mean, if someone that is 17 and 11 months old cant buy weed, then they become 18, how have they changed in any way that makes it okay for them to buy weed?